Summary Osteogenesis imperfecta (OI) is a genetically and clinically heterogeneous connective tissue disorder resulting in muscle weakness, bone deformity and increased fragility, primarily due to type I collagen gene mutations. Currently there is no cure. Genetic and clinical heterogeneity (> 1500 mutations) and growing evidence of mutation specific pathogenesis further challenges drug discovery. Deficiency in myostatin, a circulating negative regulator of muscle growth, results in increased muscle mass and physiological loading on the skeleton with concomitant increases in bone mass and biomechanical integrity. Developmental programming by the prenatal environment is hypothesized to play a significant role in lifelong bone health and promises to provide a novel modality for therapeutic intervention. Congenital myostatin null mice demonstrate both myofiber hyperplasia and hypertrophy, while mice with postnatal myostatin deficiency exhibit only hypertrophy, indicating some aspects of adult physiology are irreversibly determined during prenatal development. Previously, by three independent approaches we demonstrated that reduced maternal myostatin during pregnancy improves bone geometry and biomechanical integrity in offspring: 1) Wildtype (Wt) offspring born to dams with reduced myostatin (+/mstn) had stronger bones than Wt offspring born to Wt dams; 2) Mice with osteogenesis imperfecta (+/oim) had stronger bones when born to +/mstn dams than when born to +/oim dams; and 3) +/oim blastocysts transferred to +/mstn recipient dams had stronger bones as adults than those transferred to +/oim dams. Importantly, the last approach through embryo transfer experiments demonstrated that the maternal +/mstn effect on offspring bone was conferred by the uteroplacental environment during pregnancy. Based on these findings, we will test the efficacy of pharmacological inhibition of maternal and fetal myostatin during pregnancy via maternal anti-myostatin (GDF8) monoclonal antibody treatment as a prenatal therapeutic approach for OI using two molecularly distinct OI mouse models. The primary outcomes measures and indicators of efficacy are improved musculoskeletal health (skeletal muscle and bone mass and function) of Wt and OI offspring at the age of peak bone mass (4 month old). OI can be diagnosed prenatally, whether genetically or by ultrasound, yet we have no prenatal treatment options and, are thus missing a critical therapeutic window. This high risk study represents a more than incremental leap towards establishing the translational potential of a prenatal treatment for OI, it is a paradigm shift in understanding and treating OI; a shift from believing only genetic and postnatal factors control bone health, to the inclusion of the prenatal/perinatal period as an important and modifiable window for controlling adult bone health.